1. Technical Field
The present invention relates to an atom cell module, a quantum interference device, an electronic apparatus, and an atom cell magnetic field control method.
2. Related Art
It is known that the cesium atom, which is a kind of an alkali metal atom, has a ground level of 6S1/2 and two excitation levels of 6P1/2 and 6P3/2, as shown in FIG. 20. In addition, each level of 6S1/2, 6P1/2, and 6P2/3 has a hyperfine structure split into a plurality of energy levels. Specifically, 6S1/2 has two ground levels of F=3 and 4, 6P1/2 has two excitation levels of F=3 and 4, and 6P3/2 has four excitation levels of F=2, 3, 4, and 5.
For example, a cesium atom in the ground level of F=3 of 6S1/2 can transition to the excitation level of any of F=2, F=3, and F=4 of 6P3/2 by absorbing the D2 beam, but cannot transition to the excitation level of F=5. A cesium atom in the ground level of F=4 of 6S1/2 can transition to the excitation level of any of F=3, F=4, and F=5 of 6P3/2 by absorbing the D2 beam, but cannot transition to the excitation level of F=2. These transitions are based on the transition selection rule when an electric dipole transition is assumed. On the contrary, a cesium atom in the excitation level of one of F=3 and F=4 of 6P3/2 can transition to the ground level (either the original ground level or the other ground level) of F=3 or F=4 of 6S1/2 by emitting the D2 beam. Here, in the case of three levels (two ground levels and one excitation level) of two ground levels of F=3 and 4 of 6S1/2 and one of the excitation levels of F=3 and 4 of 6P3/2, Λ-type transition according to absorption and emission of the D2 beam is possible. Accordingly, these three levels are called Λ-type three levels. Similarly, in the case of three levels of two ground levels of F=3 and 4 of 6S1/2 and one of the excitation levels of F=3 and 4 of 6P1/2, Λ-type transition according to absorption and emission of the D1 beam is possible. Accordingly, these three levels form Λ-type three levels.
On the other hand, a cesium atom in the excitation level of F=2 of 6P3/2 always transitions to the ground level (original ground level) of F=3 of 6S1/2 by emitting the D2 beam. Similarly, a cesium atom in the excitation level of F=5 of 6P3/2 always transitions to the ground level (original ground level) of F=4 of 6S1/2 by emitting the D2 beam. That is, in the case of three levels of two ground levels of F=3 and 4 of 6S1/2 and one excitation level of F=2 or 5 of 6P3/2, Λ-type transition according to absorption and emission of the D2 beam is not possible. Accordingly, these three levels do not form Λ-type three levels. In addition, it is known that alkali metal atoms other than the cesium atom similarly have two ground levels and one excitation level that form Λ-type three levels.
Incidentally, when resonance light (assumed to be resonance light 1) having a frequency (oscillation frequency) equivalent to the energy difference between the first ground level (in the case of a cesium atom, the ground level of F=3 of 6S1/2) and the excitation level (in the case of a cesium atom, for example, the excitation level of F=4 of 6P3/2), which form the Λ-type three levels, and resonance light (assumed to be resonance light 2) having a frequency (oscillation frequency) equivalent to the energy difference between the second ground level (in the case of a cesium atom, the ground level of F=4 of 6S1/2) and the excitation level are simultaneously emitted to a gaseous alkali metal atom, a change to a superposition state of the two ground levels, that is, a quantum coherence state (dark state) is made. As a result, excitation to the excitation level is stopped. This is an electromagnetically induced transparency (EIT) phenomenon (called coherent population trapping (CPT) in some cases). The frequency difference between the resonance light pair (resonance light 1 and resonance light 2) that causes the EIT phenomenon exactly matches a frequency equivalent to the energy difference ΔE12 between two ground levels of an alkali metal atom. For example, in the case of a cesium atom, the frequency equivalent to the energy difference between the two ground levels is 9.192631770 GHz. Accordingly, the EIT phenomenon occurs when two types of laser beams of D1 and D2 beams having a frequency difference of 9.192631770 GHz are simultaneously emitted to the cesium atom.
Therefore, as shown in FIG. 21, when light with a frequency of f1 and light with a frequency of f2 are simultaneously emitted to a gaseous alkali metal atom, the two light waves become a resonance light pair. Depending on whether or not the alkali metal atom causes the EIT phenomenon, the intensity of light transmitted through the alkali metal atom is steeply changed. A signal indicating the intensity of the transmitted light that is steeply changed is called an EIT signal (resonance signal). When the frequency difference f1−f2 of the resonance light pair exactly matches a frequency f12 equivalent to ΔE12, the level of the EIT signal indicates a peak value. Therefore, a highly accurate oscillator can be realized by emitting two light waves to an atom cell (gas cell), in which gaseous alkali metal atoms are enclosed, and performing control such that the peak of the EIT signal is detected by a photodetector, that is, such that the frequency difference f1−f2 between the two light waves exactly matches the frequency f12 equivalent to ΔE12. For example, a technique relevant to such an atom oscillator is disclosed in U.S. Pat. No. 6,320,472.
Incidentally, when a magnetic field is applied to the alkali metal atom, each energy level undergoes Zeeman splitting. For example, as shown in FIG. 22A, in the case of a cesium atom, the ground level of F=3 of 6S1/2 or the excitation level of F=3 of 6P3/2 is split into seven levels corresponding to the magnetic quantum number mF=0, ±1, ±2, and ±3, and the ground level of F=4 of 6S1/2 or the excitation level of F=4 of 6P3/2 is split into nine levels corresponding to the magnetic quantum number mF=0, ±1, ±2, ±3, and ±4. In addition, the alkali metal atom causes the EIT phenomenon with two light waves, which have a frequency difference equivalent to an energy difference (frequency difference) between the Zeeman levels with the same magnetic quantum number mF at two ground levels, as a resonance light pair. That is, in a state where the magnetic field is applied to the alkali metal atom, a plurality of peaks are observed in the intensity of light transmitted through the alkali metal atom, that is, a plurality of EIT signals are observed if a frequency difference between two light waves is swept. For example, as shown in FIG. 22B, in the case of a cesium atom, seven EIT signals corresponding to the magnetic quantum number mF=0, ±1, ±2, and ±3 are observed. In general, as shown in FIG. 22B, the strength of the EIT signal corresponding to mF=0 is highest. For this reason, in many atom oscillators, a uniform steady magnetic field is applied to the gas cell, and the frequency difference between the resonance light pair is controlled so as to generate an EIT signal corresponding to mF=0. However, if the size of the atom oscillator is reduced, the volume around the gas cell is reduced. Accordingly, it is difficult to apply a stable magnetic field to the gas cell. In addition, since a certain degree of temperature is needed for the gas cell, a heater is provided. In this case, however, since a heater current is changed according to the changes in the outside air temperature, a magnetic field generated by the heater current is also changed. As a result, a magnetic field applied to the gas cell is changed by the changes in the outside air temperature. Then, as shown in FIG. 23, since the energy difference (frequency difference) between the Zeeman levels with the same magnetic quantum number mF at two ground levels changes quadratically with respect to variations of the magnetic field, a problem occurs in that the frequency stability (in particular, temperature characteristic) of the atom oscillator is degraded. In addition, if the size of the atom oscillator is reduced, the gas cell is reduced. In this case, since the total amount of atoms causing the EIT phenomenon is reduced, there is also a problem in that the strength of the EIT signal is reduced.